The present invention relates to electromechanical devices and, more particularly, to linear motors of a type adapted for driving by a direct current source.
Numerous applications require the generation of a linear force. Conventional AC and DC motors produce a rotary torque about an axis which must be converted into a linear force before it can be used in such applications. Such conversion is accomplished by a screw and nut, a sheave and cable or a rack and pinion, among others. All prior art conversion techniques require an undesirable mass and a resulting backlash which interfere with high-precision, fast-response positioning.
Linear motors which directly produce a linear force in response to an electric input are known. One such linear motor, disclosed in U.S. Pat. No. 3,376,578, takes advantage of the variable magnetic reluctance produced in the vicinity of slots in a pole face of a magnetic member. An armature of magnetic material, having windings therein, is urged to step from position to position along the pole face as defined by the slots or, alternately, the magnetic member is movable while the armature is stationary. This solution has the disadvantage of a finite step size, which limits the accuracy with which the movable element can be positioned. In addition, the inherent slowness of a stepping action is contrary to the desire for high actuation speed.
An improved linear motor having DC excitation is disclosed in U.S. patent application Ser. No. 383,351 the disclosure of which is herein incorporated by reference, wherein a slotted armature contains windings excited by varying DC voltages in the slots thereof. The armature includes a planar surface facing pole pieces of permanent magnets. The permanent magnets are affixed to a movable member such as, for example, the table of a positioning table which retains the pole pieces close to, but spaced from, the planar surface of the armature, and constrains the motion of the table and the attached permanent magnets along a linear axis parallel to the face of the armature. The DC excitation provides rapid response and, with an appropriate control system, high positioning accuracy.
Linear DC motors of the type disclosed in the foregoing referenced patent incorporate the substantial mass of the permanent magnets on the movable member. This interferes with rapid acceleration. In addition, the magnetic attraction between the poles of the permanent magnets and the magnetic material of the armature (which may reach, for example, several hundred pounds), imposes a substantial static load which must be resisted by the device supporting the movable member. Such static load, in requiring a more massive supporting member, again increases the mass to be moved, as well as imposes increased loads on the bearing members of the movable member.
The above linear DC motors are especially useful when provided with a linear commutator which energizes only those armature windings within a magnetic influence of the permanent magnets. A set of brushes, preferably carbon brushes, is affixed to move with the movable member and contact linear stationary commutator segments. The commutator segments being contacted determine which armature windings are energized. The remainder of the armature windings stay deenergized until the brushes affixed to the movable member move into their vicinity. Since only some of the armature windings are energized at any time, and the windings energized change as the movable member travels along its linear axis, the total heat-generated resistance therein is reduced and such heat is distributed over a large mass and distance from which it is more easily dissipated.
Commutation is not favored in explosive environments or in environments which cannot tolerate particulate contamination such as, for example, in clean rooms. Thus, the benefit of commutation is less easily attainable in such environments.
Commutation in the linear motor of my referenced patent performs the dual functions of enabling the armature windings within the influence of the magnets on the movable member and also feeding power to the enabled windings. The amplitude and polarity of the power fed to the selected windings is determined by a conventional control system. Other means for enabling and feeding power to windings exist without the need for commutation. For example, electro-optical, magnetic, Hall-effect and other techniques may be employed to determine the location of the movable member with sufficient accuracy to select the particular windings to be enabled. Such enabled windings may then be energized using, for example, electronic or electromechanical switches.